1. Long term Geomagnetically Induced Current Observations from New Zealand
Daniel H. Mac Manus1, Craig J. Rodger1, Mike Dalzell2, Alan W. P. Thomson3, Tim Divett1, Mark A. Clilverd4, Tanja Petersen5, Moritz Wolf6, and Neil R. Thomson1
1. Physics Department, University of Otago, Dunedin, New Zealand.
2. Transpower New Zealand Limited, New Zealand.
3. British Geological Survey, United Kingdom.
4. British Antarctic Survey (NERC), Cambridge, United Kingdom.
5. GNS Science, New Zealand.
6. Munich University for Applied Science, Munich, Germany.
“MBIE project”
ESWW14, Oostende
Friday, 01 December 2017

2. New Zealand GIC observations
Transpower New Zealand Ltd. is measuring, and archiving, observations of transformer neutral current values from DC current measuring devices (LEM) at many transformers. This has occurred from multiple South Island locations for more than 14 years.
- 36 transformers monitored in 2001
- 58 transformers monitored in 2015.
- in many substations more than 1 DC current measuring devices (LEM) is installed, independently monitoring each transformer
2012 and 2013 additions installed with Space Wx/GIC monitoring focus

3. Why so many DC observations at NZ Transformers?
New Zealand has a HVDC link to link the large hydro-generation in the South Island with large population in the North Island.
Often operations in a balanced or "bipolar" mode. But also common to operate in single wire Earth-return mode
Haywards
100% "up"
HVDC link
5-8% "stray" Earth return (multiple locations)
When in Earth-return mode, ~92-95% of the current returns directly to Benmore.
Benmore
The other 5-8% first comes into various South Island transformers and then returns to Benmore across the AC power transmission network - we term this “stray Earth return” and it is what the LEMs exist to monitor.
92-95% "direct" Earth return

4. Need to “correct” the LEM data!
100%
92-95%
5-8%
Benmore
Haywards
HVDC link
Timaru
1,108 points
=LEM
Strong linear relationship between the Timaru transformer #5 currents (TIM T5) and the total HVDC current - outside of geomagnetic storm times (EYR K<5) and for large HVDC Earth Return currents (>100A).
So can undertake weekly linear fits over all data to determine slopes, from which we can do a correction.
TIM = Timaru

5. Need to correct the LEM data!
100%
92-95%
5-8%
Benmore
Haywards
HVDC link
Timaru
=LEM
Timaru transformer #5 currents (TIM T5) data after correction.
Having removed the offsets and the stray currents from the LEM measurements, we should just have GIC present.
TIM = Timaru

6. Example of a large geomagnetic storm 6 November 2001
Eyrewell
ΔH500nT H’
Triggered by a solar wind shock observed by SOHO at 01:20 UT

7. Example of a large geomagnetic storm 6 November 2001
=LEM
HWB T4 (destroyed)
ISL SVC (tripped)
= Alarms H’
Peaks at 01:52 UT
Strong spatial variations in the peak currents over small distances!
50 km
Currents measured at ISL M6

8. Watari et al. [2009], correlations between B-field driver and Hokkaido GIC
Large GICs are usually closely associated with geomagnetic field disturbances that have a high rate of change (dB/dt) and in particular the magnetic component in the horizontal direction (dBH/dt, also represented by H') [Mäakinen, 1993; Viljanen, 1998, Bolduc et al., 1998].
However, Watari et al. [2009] concluded that in their observations "Temporal variations of GICs show high correlation with geomagnetic field variations, rather than time derivatives of the geomagnetic field"!
We test this suggestion with the NZ data.
Watari, S., et al. (2009), Space Weather, 7, S03002, doi: /2008SW

9. Case Study: Correlations for 6 Nov 2001
The highest GIC in this storm seen at Islington transformer #6 (ISL M6). So start by looking at the correlations for this storm period, and this transformer.
= GIC measuring location
ISLington
= Magnetometer
r = 0.61
ISL location is similar distance from coast as Memanbatsu
“Correlation” = Pearson r correlation coefficient
r = 0.71
Triggered by a solar wind shock observed by SOHO at 01:20 UT
Generally, rate of change wins

10. Again, rate of change wins (but clearer this time)
Case Study: Correlations for 2 Oct 2013
For consistency, look at the correlation with the GIC in this storm seen at Islington transformer #6 (ISL M6).
= GIC measuring location
ISLington
= Magnetometer
r = -0.31
“Correlation” = Pearson r correlation coefficient
Triggered by a solar wind shock observed by SOHO at 01:25 UT
r =- 0.95
Again, rate of change wins (but clearer this time)

11. General Study using Islington M6
Lets look at that at that Memanbatsu-like location again (Islington ISL M6).
= GIC measuring location
ISLington
= Magnetometer
Try and look at the long term dataset. Build a set of hourly time periods of disturbed periods. One or more of these conditions must hold:
1. Peak one minute averaged GIC magnitude is ≥10 A at anyone of ISL M6, HWB T4 HWB T6, or SDN T2, for any time during that hour.
2. variation in the EYR H is ≥200 nT
3. The peak one minute resolution value of EYR |H'| is ≥50 nT/min.
That produces 151 hourly periods for us to look into correlations at ISL M6.
“Correlation” = Pearson r correlation coefficient
Again, rate of change wins

12. St Patrick’s Day 2015 Storm – Case Study
|H’|
“HWB T4” storm
“Halloween” storm
“St Patrick’s Day” storm
Triggered by a solar wind shock observed by SOHO at 04:11 UT
For the St Patrick's Day storm the peak rate of change at EYR in the H-component maggie data is at 17-March :46 UT.
This is a particularly good event as it is in the more modern era when a large number of new current measuring locations were included!
[Cromwell starts from 2009, other start from 2012]
= New measuring locations
Cromwell
Manapouri
Clyde
Halfway Bush
South Dunedin
Invercargill

13. St Patrick’s Day 2015 Storm - Dunedin
By most international standards the St. Patrick’s Day storm response at HWB T4 (47.8 A) and SDN (45.5 A) is a large GIC event at least in the Dunedin-region, even though this is not that big a geomagnetic storm or rate of change at the storm onset.
Halfway Bush T4
In New Zealand GIC in the range A have been seen for H’ = nT/min. Dunedin is particularly responsive. Compare these observations with the effects of nT/min for 100 year extreme values [Thomson et al., 2011] and ~100 A potential failure levels [Rodger et al., Space Weather, 2017].
St. Patricks Day Storm 2015
68.6 nT/min event
HalfWay Bush
South DunediN

14. MBIE project goals
1. Understand the occurrence of GIC in the New Zealand electrical transmission network.
what is happening in space to cause the most effective GIC in the NZ network?
could this be used to provide better forecasting?
2. Test Transpower's existing GIC mitigation protocols
how well will these work?
would other approaches work better?
3. Predict the likely impact of severe/extreme geomagnetic storms in the New Zealand grid.
identify likely hotspots, changes with solar conditions.
Mitigating Emerging Risks to New Zealand's Electrical Network

15. Thankyou!
Daniel, Craig and James at the Halfway Bush (HWB) substation in front of HWB T4 which was lost on 6 November 2001 [5 May 2015].
Mac Manus, D H, C J Rodger, M Dalzell, A W P Thomson, M A Clilverd, T Petersen, M M Wolf, N R Thomson, and T Divett, Long term Geomagnetically Induced Current Observations in New Zealand: Earth return Corrections and Geomagnetic Field Driver, Space Weather, 15, 1020–1038, doi: /2017SW001635, 2017.

17. Summary HVDC Stray Currents
100%
92-95%
5-8%
Benmore
Haywards
HVDC link
By doing the corrections for all LEM sites, we can work out the significance of the stray HVDC Earth return currents Transpower believes they have instrumented all the locations where this is significant).
=LEM
Operation in Earth-return mode (ranges from 36-99% of the year from ).
Typically carries A.

18. HVDC Stray Currents Max HVDC operation is ~3000 A. 100% 5-8% 92-95%
Benmore
Haywards
HVDC link
=LEM
Max HVDC operation is ~3000 A.